Multi-DB conflict in creating new site


I have a Multi-DB setup in my multi-site and I'm having an error such as "Error establishing a database connection" when visiting the newly created site of the user. This error occured after setting up Multi-DB at my site. When I rolled back to single database setup and tested it by creating again a new site then visiting it, it was working fine. Could you please help me with this conflict?

Thank you all in advance.